Topic: Rich Carb Idle Cyl #1 PROBLEM SOLVED

[MattCB750K8]

1978 CB750 4-2 exhaust, stock motor, 20k miles, good compression and leakdown test. Valves adjusted, excellent spark etc. Stock airbox and carbs, paper oem filter. Proper voltage to ign. Properly timed and gaped points, condensers within specs. Strong spark at all 4 banks.  I purchased bike a few weeks back and do not know its history. I suspect it sat a good bit though as it only had 400 miles in 5 years. I went on a long course of tuning and maintenance. Now I have hit a stumble beyond my knowledge depth.

Running rich on cyl 1, strong fuel smell, carbon fouling plug and low exhaust temp (Measured 1" from header flange at 6").

2,3,4 all run excellent and within a few degrees with infrared gun. 1 is cold. Plugs for 2,3,4 are  good, slightly tan buff color.

Completed all ignition checks, timing, new caps, plugs etc. Confirmed by swap in 1 & 4 coil plugs. Swapped plugs also between 4-1 and the plug from #1 cleared and #4 fouled.

Plug is sooty, plug is firing, strong smell of fuel. Wet with fuel when pulled. Smoke from exhaust....NOT OIL.

Idle air screw does nothing to effect idle. As matter of fact I pulled it while running and it leaked some gas.

Checked vent tubes, over flow doesn't appear to be dumping fuel, nothing in hose and none comes out when running with tube off.

Pulled the vacuum test port and cyl leaned up and rose in temp. Idle zoomed up.  My assumption now is I am pulling fuel through either through a faulty main jet/needle or my faulty float/needle is flooding it out. I suppose a idle/main jet COULD fall of into the bowl and cause this. Sinking logged float.

I considered a faulty choke, but ruled that unlikely as its shaft is common to 1/2

Tomorrow I will check with a clear tube the true bowl level and report back and also put a vacuum gauge on the test port to see what it reads

Would a blocked idle air tube cause it to run rich enough to foul out quickly, I shy away from this as I get a little fuel from the needle plug.

I am also going to pull the filter box just to ensure the carb throat opening is not blocked for whatever reason.

Any other thoughts or ideas would be helpful. I am really try to avoid pulling the whole rack and want to eliminate everything BEFORE really messing with the 3 carbs running well. It just seems that raw fuel in being dumped in. I either have a extreme air blockage prior to the butterfly or fuel is being pulled from a leaky source.

Thanks guys

[HondaMan]

A vacuum leak in the carb hose to the head will cause overly rich mix in that cylinder, and an uneven idle. Also, a vacuum leak in the PREVIOUS cylinder in the firing order (#3 here) can also cause it. Check to make sure the hoses are still flexible (many have turned rock hard) and can still be sealed by the little band clamps. Alternately, they can be replaced, although today's Honda offerings are slightly undersized on their OD, so I have had to sub in the CB550 hose clamps in some cases, just to get them to seal with Honda's parts. Alternately, I use the Oetiker clamps (50mm size adjustables), but be careful to not overtighten these, as they can cut right thru the rubber!

Avoid the aftermarket hoses: they fail in about 2 years, cracking and splitting.

[PeWe]

Avoid the aftermarket hoses: they fail in about 2 years, cracking and splitting.

I tried cruzinimage cheap carb boots. I saw deep cracks within a few months. Crack seen from inside only.
I'll see how my Honda boots will work, the fuel is different nowadays. My bike get Shell V-power. Ethanol added here is 5%.
My old Honda boots never cracked, hard only and a reason to replace them after long time of use.

[MattCB750K8]

To this point I have used all OEM honda parts, they cost a bit more but quality of the OEM and fit makes life easier in the long run.

I will inspect all the boots carefully.

Thanks

[MattCB750K8]

Good progress and some issues found.

Hondaman: I was trying to wrap my head about if 3 is has an airleak it effects 1.  My fooling around I found that a air leak on 1 fattens up 4.  So if #2 is leaking fuel will #1 pull the leaking fuel from the airbox?

Pulled airbox and found raw fuel in #2 and #1 air inlets to carb.

Choke was partially closed on all 4 cyls. Adjusted cable and checked operation.

Ran it a little without filter box mind you, and upon shut down fuel was pouring out the main jet tube and into the air box, saturating #1 air inlet with fuel

Checked float level on all 4 carbs with clear tubing. Low and behold #2 is dumping fuel out her over flow and level is well ABOVE the bowl flange when fuel is on.

So I will be pulling the carb rack, installing new Honda carb/head hoses (Pricey little suckers) And pulling the bowls on all four, checking the fuel level needles and floats.

General cleaning off all passages etc and light rebuild

Is it possible to weight the floats to determine if they are beginning to become logged? Are they available from honda if needed.? They seem to be of the same material as the old q-jets and the where notorious for sinking over time making dry measurements useless.

Does honda still have the carb kits available? Are the aftermarket kits of high quality, especially the rubbers and needle seat seals?

Thanks for the ideas.

[HondaMan]

Good progress and some issues found.

Hondaman: I was trying to wrap my head about if 3 is has an airleak it effects 1.  My fooling around I found that a air leak on 1 fattens up 4.  So if #2 is leaking fuel will #1 pull the leaking fuel from the airbox?

Pulled airbox and found raw fuel in #2 and #1 air inlets to carb.

Choke was partially closed on all 4 cyls. Adjusted cable and checked operation.

Ran it a little without filter box mind you, and upon shut down fuel was pouring out the main jet tube and into the air box, saturating #1 air inlet with fuel

Checked float level on all 4 carbs with clear tubing. Low and behold #2 is dumping fuel out her over flow and level is well ABOVE the bowl flange when fuel is on.

So I will be pulling the carb rack, installing new Honda carb/head hoses (Pricey little suckers) And pulling the bowls on all four, checking the fuel level needles and floats.

General cleaning off all passages etc and light rebuild

Is it possible to weight the floats to determine if they are beginning to become logged? Are they available from honda if needed.? They seem to be of the same material as the old q-jets and the where notorious for sinking over time making dry measurements useless.

Does honda still have the carb kits available? Are the aftermarket kits of high quality, especially the rubbers and needle seat seals?

Thanks for the ideas.

For the '78 you can get carb kits from places like PartsNmore, but avoid using their needles and jets, if at all possible. They are not the same as yours. I just bought 4 sets of "O-rings only" and 4 bowl gaskets from them last month, not very expensive, worked great.

The floats don't get waterlogged or anything like that.

The "subsequent cylinder rich" phenomenon is due to these carbs having to mix richer at lower speeds, because the pulse-draw is very short. So, if the cylinder ahead of the one you are working on stumbles, the engine slightly slows down for the next one in the firing order. Since it then momentarily runs slower, the airflow thru that carb is slower, and this makes it run richer than the carb "ahead" of it. This is true of all of them in the firing order.

The phenomenon you saw with #4 is probably due to poorly-sealing hoses all around.

[MattCB750K8]

Haven't made much progress this week, I have high dose radiation for my cancer the next 4 weeks. I did manage to pull the carbs and setup for their overhaul.

Accel pump seemed to work well a squirted the carbs evenly.

I pulled the floats and see no issues, so I weighed them and the are all 9g.

An interesting note on the fuel needles was the springs are of all different tensions. Hence when turned upside down the floats sat at different levels and had various degrees of travel on the float spring.

I did get OEM Honda Float needles for all 4.

I did get new Gasket kits, plus I ordered new low speed jets from Sirus, new main 110 jets as I cannot measure the old 100%.

I dont think I will unrack them, everything, think I can do a good clean and overhaul without seperation

The issue I have now is the carb holders, I can't find new OEM Hondas. I don"t like aftermarket rubber for the most part. In general I have always found it perishes very quickly. All my clamps looked suspect to so I ordered new ones also.

Ideas for new OEM Honda K8 holders?

[flybox1]

Haven't made much progress this week, I have high dose radiation for my cancer the next 4 weeks. I did manage to pull the carbs and setup for their overhaul.

Accel pump seemed to work well a squirted the carbs evenly.

I pulled the floats and see no issues, so I weighed them and the are all 9g.

An interesting note on the fuel needles was the springs are of all different tensions. Hence when turned upside down the floats sat at different levels and had various degrees of travel on the float spring.   This is why the clear tube test has merit and needs to be done to ensure you have adequate fuel in the bowls.

I did get OEM Honda Float needles for all 4.

I did get new Gasket kits, plus I ordered new low speed jets from Sirus, new main 110 jets as I cannot measure the old 100%.

I dont think I will unrack them, everything, think I can do a good clean and overhaul without seperation  No need to separate them

The issue I have now is the carb holders, I can't find new OEM Hondas. I don"t like aftermarket rubber for the most part. In general I have always found it perishes very quickly. All my clamps looked suspect to so I ordered new ones also.

Ideas for new OEM Honda K8 holders?  Aftermarket might be your best option.

[MattCB750K8]

Issue Found I believe

The Idle Air screw was buggered buy the previous owner either had a monkey or was a monkey working on the carbs.
Disassembled #1, removed the idle air screw, there was no washer and bits and pieces of 2 o-rings. Pieces of the o-ring was jammed into the air passages. I used a combination of air and Guitar string to clear the passages through out the carb.

The low speed jet was also a bit buggered, It was actually bent and the small holes a bit restricted. Cleared them, but I have new low speeds so will probably use them.

Main jets/emulsion tubes looked very good.

Inspection of the remainder Idle Air Screws found the O-rings and washers in place but the O-rings badly compressed and dried.

With a good careful cleaning, new low speed jets (stock #35), new float needles and gaskets and a proper float set I should be back in business.

I did get new carb holders and intake boots so I will probably renew them and the clamps.

As I recall its 14.5mm on float height, just contacting the float needle spring but not compressing. 1.5 turns out to start on the Idle Air screws.

[flybox1]

Excellent. 
the IMS stack is [///////]spring/washer/o-ring>---

(reverse the o-ring and washer, and it will crumble under adjustment)
Keep us posted on how it runs.

[MattCB750K8]

Finally feeling better and took a little time on the bike.


Overhauled the carbs and got them back on. Although I didn't hook anything up but the holders and fuel line.  Fired right up WITH CHOKE, set the idle up and let it clear #1 pipe...bit smokey and popping for a few minutes. cleared up and all cyl are now firing within 4 or 5 degrees in temp. I will fine tune it a bit when I get the airbox and cables back on and everything fully set.

Going to change oil again for good measure.

Thanks for all the input.

[flybox1]

Glad its better.  Probably popping because its too lean without airbox. 
Bike will always behave and idle better with airbox on...it needs that flow resistance to maintain the proper air/fuel mixture.

[HondaMan]

Finally feeling better and took a little time on the bike.


Overhauled the carbs and got them back on. Although I didn't hook anything up but the holders and fuel line.  Fired right up WITH CHOKE, set the idle up and let it clear #1 pipe...bit smokey and popping for a few minutes. cleared up and all cyl are now firing within 4 or 5 degrees in temp. I will fine tune it a bit when I get the airbox and cables back on and everything fully set.

Going to change oil again for good measure.

Thanks for all the input.

Good job!
Those teensy O-rings in the '77-'78 PD carbs' air screws are often broken, causing things similar to this.

The PD42 carbs came in an "A" and a "B" version. The differences between them are: the "B" (tight emission controlled type) have a #35 pilot jet and 14.5mm float depth, makes them run lean and are quite cold-blooded. The "A" type will have either a #35 or #38 pilot jet (it is actually a #37.5, but part # = 38), I've seen both, but the float level is a slightly richer 12.5mm. This reduces the cold-bloodedness noticeably. These carbs run so lean that they are pretty touchy about being balanced closely (i.e., vacuum), and if they are 'off' a little they will pop, often during decel or at idle. If you can figure out which one is popping, try adjusting its balancer screw about 1/4 turn (one way, then the other if it gets worse) to make it hush back up. That's then mighty close to touring tune!

[MattCB750K8]

Once I got her all buttoned up with all new carb boots and clamps the popping stopped and exhaust temp dropped about 30 degrees. Sync seems good although I have not put the gauges to them yet.

I have a high end IR temp gun. Normally on reflective surfaces I paint a black dot or stripe. But in this case I am checking across for disparity between cyls. All are within a few degrees at idle and various  throtttle, idle positions. For off idle condition I use 2000-2500 as a reference as I recall advance is either coming in or fully in and can then be taken into consideration.
For the idle circuit I am letting it idle around 1000.

Yes it seems cold blooded for the first couple of minutes. I Choke, Start wait a minute then 1/2 choke, then a minute or two later choke off, she really smooths out after 3 or 4 min.

When I get some energy back I will take her out for a 20 mile run and pull the plugs for a main jet/float height assessment .

Funny, this my first time working with any form of carbs at all, not to difficult but boy are the passages and jets TINY.

[flybox1]

Yes it seems cold blooded for the first couple of minutes. I Choke, Start wait a minute then 1/2 choke, then a minute or two later choke off, she really smooths out after 3 or 4 min.

Sounds exactly like my K8....I start and immediately ride 2 blocks the first light and take it to half-choke when stopped.  When I go on green, the choke goes off and I dont need to touch it again.

[madmtnmotors]

Yes it seems cold blooded for the first couple of minutes. I Choke, Start wait a minute then 1/2 choke, then a minute or two later choke off, she really smooths out after 3 or 4 min.

Sounds exactly like my K8....I start and immediately ride 2 blocks the first light and take it to half-choke when stopped.  When I go on green, the choke goes off and I dont need to touch it again.

Yep, my 78 750's as well. Full choke for a couple minutes and let the choke off as the idle starts to rise little by little until it idles smooth with no choke.